The invention relates to Doppler frequency estimation, and more particularly, to receiving spread spectrum radio signals, such as digitally modulated signals in a Code Division Multiple Access (CDMA) mobile radio telephone system.
In a CDMA system, different users, base stations, and services are usually separated by unique spreading sequences/codes. The rate of the spreading code (usually referred to as the chip rate) is larger than the information symbol rate. The code rate divided by the information symbol rate is usually referred to as the spreading factor (sf). An information data stream is spread (or coded) by multiplying the information data stream with the spreading code. In a system where there are multiple users, coded signals can be added together to form a composite signal. A receiver can recover any one of the information data streams by correlating the composite signal with the conjugate of the corresponding spreading code.
In a mobile communications system, signals transmitted between base and mobile stations typically suffer from echo distortion or time dispersion (multipath delay). Multipath delay is caused by, for example, signal reflections from large buildings or nearby mountain ranges. The obstructions cause the signal to proceed to the receiver along not one, but many paths. The receiver receives a composite signal of multiple versions of the transmitted signal that have propagated along different paths (referred to as “rays”). In order to optimally detect the transmitted signal, a device known as a searcher finds the different rays, and another device known as a RAKE receiver “rakes” them together.
FIG. 1 is an illustration of an exemplary frame structure in a CDMA system. Frame 100 has multiple slots 101, 102, 103, 104, . . . , 116. Each slot has a pilot portion 120 and a data portion 130. It will be evident to those skilled in the art that different CDMA systems may have different frame structures. In the example shown, the pilot portion 120 has four pilot symbols 121, 122, 123, and 124, and the data portion 130 has multiple data symbols 131, 132, 133, 134, . . . n. The pilot symbols 121, 122, 123, and 124 can be used to find different rays. Because the pilot symbols are known at the receiver, the searcher can use a filter that is matched to the pilot symbols (a matched filter) to find the different paths. The output of the matched filter is usually referred to as the multipath profile (or the delay profile). Because the received signal contains multiple versions of the same signal, the delay profile contains more than one spike. The different spikes correspond to the different rays. As discussed more fully below, the pilot symbols 121, 122, 123, and 124 can also be used for channel estimation.
FIG. 2 is a schematic diagram of the searcher and RAKE receiver portions of a receiver. A transmitter (not shown) transmits a signal to receiver 200. Because the signal travels along multiple paths, received signal 201 contains multiple versions of the same signal. Searcher 300 uses a matched filter 310 and a peak detector 350 to find and select a set of strongest rays. Searcher 300 can use a second matched filter 320, a slot delay 330, and an accumulator 340 to search more than one slot of frame 100.
RAKE receiver 400 has six fingers 410, 420, 430, 440, 450, and 460. Each finger is a simple receiver that is configured to receive a different path of the signal 201. For example, finger 410 is configured to receive a path having a time delay of td1. Fingers 420, 430, 440, 450, and 460 are configured to receive paths having time delays of td2, td3, td4, td5, and td6, respectively. The outputs of fingers 410, 420, 430, 440, 450, and 460 are multiplied by individual weights to maximize the received signal-to-noise-and-interference ratio. The weighted outputs are then added by an accumulator 700. The output of accumulator 700 is fed to a detector 800.
Suppose searcher 300 finds a set of rays, but that receiver 200 is a mobile hand-held unit. As receiver 200 moves, these rays are no longer the best rays. If receiver 200 uses weak rays, the signal quality will decrease. The only way that receiver 200 can maintain the same signal quality is to request additional signal power from the base station. Additional signal power increases the amount of interference experienced by the other receivers. The overall interference is minimized when each receiver uses the least amount of signal power possible.
Searching for new rays is computationally complex. It is not only time-consuming, it also decreases the battery life of the receiver. The need to search for new paths (and the time delay of the new best paths) is largely dependent on the relative velocity of the receiver. If the receiver can determine the Doppler frequency of the mobile, the receiver can determine whether the mobile has moved and whether searching for new paths is necessary. The receiver can also use the Doppler frequency to track or predict new paths. While researchers have long recognized Doppler frequency as one of the phenomenons affecting the radio channel, these researchers have not developed an effective method for deriving the Doppler frequency from the received signal itself. There is a need for a simple and reliable way to determine the Doppler frequency of a mobile station.